In this case ;-), I think a simple CASE will do exactly the same thing:

SELECT TOP 1

CASE WHEN Col1 IS NULL THEN 1 WHEN Col2 IS NULL THEN 2 WHEN Col3 IS NULL THEN 3 WHEN Col4 IS NULL THEN 4

WHEN Col5 IS NULL THEN 5 WHEN Col6 IS NULL THEN 6 WHEN Col7 IS NULL THEN 7 WHEN Col8 IS NULL THEN 8

WHEN Col9 IS NULL THEN 9 WHEN Col10 IS NULL THEN 10 WHEN Col11 IS NULL THEN 11 WHEN Col12 IS NULL THEN 12 END

FROM @T